1. Field of the Invention
The present invention relates in general to a differential voltage transmission circuit. In particular, the present invention relates to a differential voltage transmission circuit with adjustable reference current.
2. Description of the Related Art
FIG. 1 shows a conventional differential voltage transmission circuit. The conventional differential voltage transmission circuit comprises a bandage reference circuit 10 to provide a bandgap bias Vbg. The reference current Iref is generated by the bandgap bias Vbg and the external resistor Rext. Next, the reference current Iref is output to transmitter 12. The transmitter 12 comprises an external termination resistor Rterm. When the reference current Iref flows through the external termination resistor Rterm, a first output voltage Vop and a second output voltage Von are generated at both terminals of the termination resistor Rterm, then the output voltage Vo is generated, that is, the difference between the first output voltage Vop and the second output voltage Von.
Because the reference current Iref is influenced by the external resistor Rext, the value of the external resistor Rext must be very exact to avoid influencing the reference current Iref. Thus, the external resistor Rext is fabricated by another factory, after confirming the actual resistance, then the external resistor Rext is set on the circuit board to generate an exact reference current Iref.
However, the circuit board must add via holes to set up the external resistor Rext. The additional external resistor Rext and via holes, however, increase costs and complicate the fabricating process.
To solve the cost problem, the external resistor Rext is replaced with an internal resistor formed with the bandage reference circuit 10 by CMOS process. Thus, the cost of the external resistor Rext is economized. However, errors with the internal resistor formed by CMOS process are increased to as much as xc2x130%, which cannot provide a demanding reference current.
The object of the present invention is to provide a differential voltage transmission circuit that provides the reference current by adjustment without using the external resistor.
To achieve the above-mentioned object, the differential voltage transmission circuit comprises a reference bias circuit, a differential comparator, a decision circuit, and an output driving circuit. The reference bias circuit outputs a first reference voltage, a second reference voltage and a reference current corresponding to a reference current adjusting signal. The differential comparator compares the difference between the first reference voltage and the second reference voltage with the difference between a first output voltage and a second output voltage, and outputs a result signal corresponding to the compared result. The decision circuit outputs the reference current adjusting signal corresponding to the result signal. The output circuit outputs the first output voltage and the second output voltage generated at both terminals of a termination resistor when the reference current flows through the termination resistor.